There has been known an image forming apparatus which includes a photosensitive drum configured to form an electrostatic latent image thereon, a developing device configured to supply toner to the electrostatic latent image on the photosensitive drum and to form a toner image on the photosensitive drum, a transfer roller configured to transfer the toner image on the photosensitive drum to a sheet, and a cleaning roller configured to collect the toner remaining on the photosensitive drum without being transferred to the sheet (refer to JP-A-H9-197841). This image forming apparatus executes a holding control of applying to the cleaning roller a constant holding bias having an opposite polarity to the toner such that the cleaning roller collects and holds thereon the toner on the photosensitive drum, and a collection control of applying to the cleaning roller a constant discharge bias having the same polarity as the toner such that the toner on the cleaning roller is discharged to the photosensitive drum and the developing device collects the toner via the photosensitive drum.
However, in this image forming apparatus, since the discharge bias is set to be constant, when a relatively large amount of toner is held on the cleaning roller, a discharge amount of the toner is large at the first one rotation of the cleaning roller from a time at which the collection control starts, and the discharge amount of the toner gradually decreases thereafter. That is, a density difference of the discharged toners would become larger. In this case, the developing device may not be able to favorably collect the toner having the large density difference on the photosensitive drum unless the developing device is designed to cope with the phenomenon.